The signals used in transmitting information via radio teleprinter channels are an example of complementary series of pulses. These signals are known as "mark" and "space" signals and are complementary because when the mark signal ends, the space signal begins and both the mark and space signals contain the same transmitted information. The radio transmitter emits power during both the mark and space signals but shifts the frequency of the transmitted signal slightly to generate mark and space radio signals at radio frequencies.
A radio receiver produces mark and space signals at two audio frequencies havng a predetermined frequency shift between the mark and the space audio signals. The terminal unit detects information from both the mark and space signals simultaneously.
More particularly, terminal units of the prior art receive mark and space signals from a radio receiver in the form of audio tones at two different frequencies with the mark signals having one frequency such as 2125 Hz and the space signals having another frequency such as 2975 Hz, for example. The mark and space tones are processed in separate channels which include a filter in each channel, with one filter designed to pass only tones at the frequency of the space signals and the other filter designed to pass only tones at the frequency of the mark signals. The tones passed by each filter are then coupled to a detector for demodulating, the tones thus resulting in a series of DC signals. The mark and space signals from the detectors are then summed. The combined signals may be coupled either directly or indirectly to an output device such as a teleprinter or a cathode ray tube for converting the series of DC output signals to a visual presentation of alphanumeric characters to a reader.
In some terminal units of the prior art, lamps are used to indicate the reception of mark and space signals. Such lamps are either on or off and do not provide a real indication of the magnitude of the mark and space signals. They are therefore unsatisfactory for use in tuning a radio receiver to a precise frequency for producing the maximum possible signal level in both the mark and space channels. In other terminal units of the prior art, a cathode ray tube (CRT) is used as the tuning indicator. Conventionally, the mark signal is displayed by the horizontal trace of the cathode ray tube and the space signal by the vertical. Thereby presenting a cross pattern display of the mark and space signals to the operator. Although a cathode ray tube does provide an indication of the instantaneous magnitude of the mark and space signals, it is subject to several disadvantages. The cathode ray tube is becoming scarce and quite expensive to purchase. Further, it requires a high voltage power supply, which includes a number of components and is itself expensive. The cathode ray tube requires a warm-up period after the terminal unit is energized before the tube provides any indication of the mark and space signals. Because the cathode ray tube displays not only the information contained in the mark and space signals but also lines caused by the retrace of the beam of the cathode ray tube, weak signals and signals with a low signal to noise ratio are difficult to properly tune in. These types of signals become lost in the "ball of noise" of the retrace lines.
Also, a cathode ray tube display is difficult for an operator to view and interpret if the operator is more than approximately ten feet from the cathode ray tube. If the radio receiver and terminal unit are installed in a large room, an operator viewing a cathode ray tube indicator may not simply glance across the room to observe the amplitude of the mark and space signals, but must move sufficiently close to the cathode ray tube to do this. Furthermore, the failure rate of cathode ray tubes is rather high compared to other components in a terminal unit. In addition, there is a degradation in the performance of the cathode ray tube with increasing age of the tube or extensive use, because of the duty-cycle, i.e. the tube is normally used to display signals only during a small proportion of the time that it is energized.